Why are lemons sour?
How do acids act?
Some acids are very powerful. There is one, called _hydrofluoric
acid_, that will eat through glass and has to be kept in wax bottles;
and all acids tend to eat or corrode metals. You saw what hydrochloric
acid did to the zinc shavings when you wanted to make a balloon; or,
to be more accurate, you saw what the zinc shavings did to the acid,
for the hydrogen gas that bubbled off was driven out of the acid by
the zinc. Then the zinc combined with the rest of the acid to form
what chemists call a _salt_.
If we were to let the soft metal, sodium, act on hydrochloric acid,
we should get hydrogen also; but the salt that formed would be regular
table salt (NaCl). You cannot do this experiment, however, as the
sodium does its work so violently that it is dangerous.
EXPERIMENT 105. _To be done by the teacher before the class.
If acid spatters on any one's skin or clothes, wash it of
immediately with ammonia or a strong soda solution._
Drop a little candle grease on a piece of copper about 3/4
inch wide and 2 or 3 inches long. In the flame of a Bunsen
burner, gently heat the end of the copper that has the candle
grease (paraffin) on it, so that the paraffin will spread out
all over the end. Let it harden. With a nail, draw a design in
the paraffin on the copper, scratching through the thin coat
of paraffin to the copper below. Pour a couple of drops of
concentrated nitric acid on the paraffin-covered end of the
piece of copper, and spread the acid with a match so that
it can get down into the scratches. Let it stand by an open
window for 5 or 10 minutes. Do not inhale the brown fumes
that are given off. They are harmless in small amounts, but if
breathed directly they are very irritating. Now wash off the
acid by holding the copper under the hydrant, and scrape off
the paraffin.
[Illustration: FIG. 182. Etching copper with acid.]
The nitric acid did to the copper in this experiment exactly what
the hydrochloric acid did to the zinc shavings when you made the toy
balloon. The copper drove the hydrogen out of the nitric acid and
incidentally broke down some of the nitric acid to make the brown gas,
and then the copper joined the rest of the nitric acid to make a
salt called _copper nitrate_. This salt is green, and it dissolves in
water. When you washed the copper, the green salt was washed away and
a dent remained in the copper where the copper salt had been.
Here is a more practical experiment showing the action of acid on
metal:
EXPERIMENT 106. Use two knives, one of bright steel and the
other a silver-plated one. If the steel knife is not bright,
scour it until it is. Drop a little lemon juice on each knife
and let it stand for a few minutes, while the teacher does the
next experiment. Then rinse both knives and examine them. What
has the lemon juice done to the silver knife? to the steel
one?
The lemon juice acts in this way because it is acid. Acids act on the
taste nerves in the tongue and give the taste of sourness; everything
sour is an acid. The black stuff formed on the steel knife by the
lemon juice is an iron salt. The iron in the knife drove the hydrogen
out of the lemon juice, but there was not enough for you to see it
coming off; then the iron combined with the rest of the lemon juice to
form an iron salt.
Whenever an acid acts on a metal, the metal drives off the hydrogen
and forms a salt. The salt is not always good to eat; for instance,
the salt that tin forms with acids is poisonous.
ACTION OF ACIDS ON OTHER SUBSTANCES. Acids do not act on metals only,
however. Watch the next experiment to see what a strong acid will do
to cloth.
EXPERIMENT 107. _To be done by the teacher._ Put a drop of
concentrated nitric or sulfuric acid on a piece of colored
wool cloth, or on a piece of colored silk. Let it stand for
a few minutes, then rinse it thoroughly. Test the cloth where
the acid has been to see whether or not it is as strong as the
rest of the cloth. How has the acid affected the color?
[Illustration: FIG. 183. Strong acids will eat holes like this in
cloth.]
ACTION OF ACIDS ON THE NERVES OF TASTE. Acids act on the taste nerves
in the tongue and give the taste of sourness; everything sour is an
acid. Lemon juice, sour milk, and sour fruits are all too weak acids
to injure clothes or skin, but their sour taste is a result of the
acid in them acting on the nerves of taste.
_APPLICATION 79._ A girl wanted to make lemonade. She did
not know which of two knives to use, a steel-bladed one or a
silver-plated one. Which should she have used?
_APPLICATION 80._ A woman was going to put up some tomatoes.
She needed something large to cook them in. She had a shiny
new tin dish pan, an older enamelware dish pan, a galvanized
iron water pail, and an old-fashioned copper kettle. Which
would have been best for her to use?
Make a list of as many acids as you can think of.
INFERENCE EXERCISE
Explain the following:
511. Sugar dissolves readily in _hot_ coffee.
512. The sugar disappears, yet the coffee flavor remains and
so does the sweetness of the sugar.
513. A tin spoon left overnight in apple sauce becomes black.
514. If one's clothes are on fire, rolling over on the ground
is better than running.
515. Lemon juice bleaches straw hats.
516. Will-o'-the-wisps glow at night, deceiving travelers by
their resemblance to moving lanterns.
517. Tomatoes should never be left in a tin can after it has
been opened.
518. Boiled milk tastes different from ordinary milk.
519. Your hands become very cold after you have washed things
in gasoline.
520. Wood decays more quickly when wet than when dry.